Techniques are known in wireless communication systems for locating a mobile subscriber unit, such as a hand held radio telephone or vehicle radio telephone in a radio telephone system, such as a cellular telecommunication system. In a current CDMA communication system governed by standard IS-95, a broadcast channel transmits a pilot offset index field, a base site latitude and longitude field and a list of neighboring cells. Therefore it is possible that a mobile subscriber unit can calculate its location on a latitude and longitude map using time difference of arrival (TDOA) hyperbolic coordinates if there are enough base stations within a receive range of a mobile subscriber unit.
However, a problem arises with such a system since the base station typically only devotes at most about 20% of its transmit power to the pilot sub channel. Moreover, since the CDMA system employs a spread spectrum channel coding for traffic channels at a time and frequency using multiple spreading codes, the traffic channels add a great deal of noise to the time of arrival measurements. The pilot sub channels are all transmitted continuously to a mobile subscriber unit. Generally, each base station repeats a pilot sub channel at different time intervals and the mobile subscriber unit knows the differing time intervals from pre-stored information or data sent in the broadcast channel. A mobile subscriber unit uses the pilot channel to synchronize with traffic channels. Hence a pilot sub channel is broadcast providing timing and phase synchronization to aid in subsequent demodulation of a transmitted signal. With the traffic channel noise and the low power of the pilot sub-channels, it is difficult for mobile subscriber units to perform trilateration over a large percentage area of a cellular coverage area
Another known system for determining mobile subscriber unit location is to allow the mobile subscriber unit to serve as a beacon so neighboring base sites can determine a location of the mobile through trilateration techniques. The mobile subscriber unit transmits a power up function and the base sites attempt to determine a location of the mobile unit using data from three or more base sites in an area. A problem with using a mobile subscriber unit as a location beacon is that mobile subscriber units typically tend to have low power output capabilities, some as low as 0.2 watts, which does not typically afford a strong enough signal for accurate location determination. Also, there is a substantial amount of noise since mobiles are communicating the power up f unction at specified frequencies that are reused by multiple mobiles within a given coverage area.
Moreover in traditional CDMA telecommunication systems, control channels, traffic channels and the pilot subchannels are transmitted by a base station on the same frequency using different spreading codes. Therefore the noise from the traffic channel and control channels can reduce the detection abilities of mobile subscriber units to determine locations of base sites.
Consequently there exists a need for a spread spectrum channel coding system that facilitates location of mobile subscriber units by enabling a mobile subscriber unit to make required measurements for autonomously locating itself.